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109 lines
3.6 KiB
C++
109 lines
3.6 KiB
C++
//===-- llvm/MC/MCFixup.h - Instruction Relocation and Patching -*- C++ -*-===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_MC_MCFIXUP_H
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#define LLVM_MC_MCFIXUP_H
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#include <cassert>
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namespace llvm {
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class MCExpr;
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// Private constants, do not use.
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//
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// This is currently laid out so that the MCFixup fields can be efficiently
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// accessed, while keeping the offset field large enough that the assembler
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// backend can reasonably use the MCFixup representation for an entire fragment
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// (splitting any overly large fragments).
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//
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// The division of bits between the kind and the opindex can be tweaked if we
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// end up needing more bits for target dependent kinds.
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enum {
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MCFIXUP_NUM_GENERIC_KINDS = 128,
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MCFIXUP_NUM_KIND_BITS = 16,
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MCFIXUP_NUM_OFFSET_BITS = (32 - MCFIXUP_NUM_KIND_BITS)
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};
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/// MCFixupKind - Extensible enumeration to represent the type of a fixup.
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enum MCFixupKind {
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FK_Data_1 = 0, ///< A one-byte fixup.
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FK_Data_2, ///< A two-byte fixup.
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FK_Data_4, ///< A four-byte fixup.
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FK_Data_8, ///< A eight-byte fixup.
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FirstTargetFixupKind = MCFIXUP_NUM_GENERIC_KINDS,
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MaxTargetFixupKind = (1 << MCFIXUP_NUM_KIND_BITS)
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};
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/// MCFixup - Encode information on a single operation to perform on an byte
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/// sequence (e.g., an encoded instruction) which requires assemble- or run-
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/// time patching.
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///
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/// Fixups are used any time the target instruction encoder needs to represent
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/// some value in an instruction which is not yet concrete. The encoder will
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/// encode the instruction assuming the value is 0, and emit a fixup which
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/// communicates to the assembler backend how it should rewrite the encoded
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/// value.
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///
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/// During the process of relaxation, the assembler will apply fixups as
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/// symbolic values become concrete. When relaxation is complete, any remaining
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/// fixups become relocations in the object file (or errors, if the fixup cannot
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/// be encoded on the target).
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class MCFixup {
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static const unsigned MaxOffset = 1 << MCFIXUP_NUM_KIND_BITS;
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/// The value to put into the fixup location. The exact interpretation of the
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/// expression is target dependent, usually it will one of the operands to an
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/// instruction or an assembler directive.
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const MCExpr *Value;
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/// The byte index of start of the relocation inside the encoded instruction.
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unsigned Offset : MCFIXUP_NUM_OFFSET_BITS;
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/// The target dependent kind of fixup item this is. The kind is used to
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/// determine how the operand value should be encoded into the instruction.
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unsigned Kind : MCFIXUP_NUM_KIND_BITS;
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public:
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static MCFixup Create(unsigned Offset, const MCExpr *Value,
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MCFixupKind Kind) {
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MCFixup FI;
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FI.Value = Value;
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FI.Offset = Offset;
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FI.Kind = unsigned(Kind);
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assert(Offset == FI.getOffset() && "Offset out of range!");
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assert(Kind == FI.getKind() && "Kind out of range!");
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return FI;
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}
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MCFixupKind getKind() const { return MCFixupKind(Kind); }
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unsigned getOffset() const { return Offset; }
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const MCExpr *getValue() const { return Value; }
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/// getKindForSize - Return the generic fixup kind for a value with the given
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/// size. It is an error to pass an unsupported size.
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static MCFixupKind getKindForSize(unsigned Size) {
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switch (Size) {
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default: assert(0 && "Invalid generic fixup size!");
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case 1: return FK_Data_1;
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case 2: return FK_Data_2;
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case 4: return FK_Data_4;
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case 8: return FK_Data_8;
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}
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}
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};
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} // End llvm namespace
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#endif
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